This proposal requests support for a continuing program which will examine the development of the axon membrane in normal and pathological nerve fibers, and the relationship of axon membrane differentiation to glial association and myelination. Over the past two years, our studies have led to progress in the following areas: i) physiology of developing CNS myelinated fibers; ii) axon membrane ultrastructure of developing central myelinated fibers; iii) axonal and axolemmal development in a glial deficient environment; iv) axo-glial relationships during myelination ; v) physiology of regenerating axons; vi) physiology of demyelinated fibers; vii) physiological differentiation of motor and sensory axons; viii) conduction properties of inhomogeneous fibers. One of the major results of these studies is the demonstration of a structural reorganization of the axon membrane during the development of myelinated fibers. To date, the relationship between this axon membrane differentiation and axo-glial association has remained incompletely studied. We now plan to examine the differentiation of the axon membrane, and the dependence of this differentiation on glial association, in developing optic nerve and spinal cord. These studies will utilize several complementary techniques (freeze-fracture, cytochemistry, electrophysiology, H3 saxitoxin binding) to examine axon membrane development both in normal axons, and in fibers which have developed in a glial cell deficient environment. We plan, in these studies, to examine the sequence of events in the differentiation of the axonal membrane of myelinated fibers, and to determine whether (and if so, with what time course and characteristics) nodal and internodal membrane can develop in response to delayed ensheathment with glial cells. We also plan to investigate the relationship between development of H3 STX binding, and freeze-fracture ultrastructure of the axolemma. These studies will provide information about the normal development of myelinated axons, and the characteristics of axons which are deprived of glial contact. We believe that these results will be relevant to understanding mechanisms of membrane plasticity which may be important in the demyelinating diseases.